1. Field of the Invention
This invention relates to doping of group III-V semiconductor compounds and, more specifically, to improvements in doping stability of carbon-doped group III-V semiconductor compounds.
2. Brief Description of the Prior Art
Recently, carbon has become a widely used p-type dopant in organometallic vapor-phase epitaxy (OMVPE, also often called MOVPE, MOCVD and OMCVD) of GaAs. Due to its low diffusivity, even at high concentrations, carbon is an ideal dopant for the device structures which require heavily doped p-type layers such as AlGaAs/GaAs heterojunction bipolar transistors (HBT) and vertical field-effect transistors (VFET). This is set forth in an article of B. T. Cunningham et al., Applied Physics Letters, Vol 54, pg. 1905 (1989).
Two most widely used methods of carbon doping in OMVPE are (1) adding CCl.sub.4 as an intentional dopant source and (2) using trimethylarsine ((CH.sub.3).sub.3 As or Me.sub.3 As) as an arsenic and carbon source without any intentional dopant source. These are set forth in articles of P. M. Enquist, Applied Physics Letters, Vol. 57, pg. 2348 (1990) and T. Kobayashi et al., Journal of Crystal Growth, Vol. 102, page 183 (1990). Both methods can yield hole concentrations in excess of 1.times.10.sup.20 /cm.sup.3 in GaAs, but the doping concentration depends upon many growth parameters, such as growth temperature, ratio of group III to group V elements and growth rates. In the second method, the growth temperature is the most convenient parameter to vary the doping concentrations, as lower growth temperatures yield higher doping concentrations. When CCl.sub.4 is used, the doping concentration can be varied to a certain extent by changing the CCl.sub.4 flow, but to minimize the CCl.sub.4 flow, which is potentially harmful to the reactor, lower growth temperatures (.ltoreq.600.degree. C.) are desirable for higher doping concentrations. On the other hand, using low growth temperatures (.ltoreq.600.degree. C.) is not desirable since the electrical and optical properties of carbondoped GaAs (GaAs:C) degrade after heat treatment. Annealing of GaAs:C reduces the hole concentration and mobility and photoluminescence intensity as set forth in an article by M. C. Hanna et al., Applied Physics Letters, Vol. 59, page 2001 (1991). Such a degradation is particularly troublesome for heavily carbon-doped GaAs since growth temperatures used for such high doping levels may not be high enough for the growth of subsequent layers. For example, growing an AlGaAs emitter layer after a heavily carbon-doped base layer of an AlGaAs/GaAs HBT may require raising the growth temperature substantially, the elevated temperature being sufficiently high to degrade the electrical properties of the base layer. Also, the device fabrication procedure frequently requires high temperature processing steps, such as overgrowth and annealing. These processing steps can also degrade the electrical properties of the base layer.